Add wp-l10n-validator as a composer dependency

See #11
Have Tavis CI run the l10n validator

See #11
Use correct path to l10n validator

See #11
Don't syntax check PHP files in ./vendor

See #11, JDGrimes/wp-l10n-validator#9
Update l10n validator config

See #11
1.8.0-alpha-1
Merge branch 'issue-11'